Subterranean hydrocarbon reservoir oil is typically characterized in terms of four compositional fractions—saturates (saturated hydrocarbons), aromatics, resins, and asphaltenes. Asphaltenes may be defined as the organic part of crude oil that is not soluble in straight-chain solvents such as pentane or heptane. Asphaltenes that are soluble in the bulk oil at relatively high pressures may precipitate when pressure drops below a certain value, called the onset pressure. As reservoir pressure decreases and drops below the onset pressure during hydrocarbon production, asphaltenes may precipitate then flocculate and deposit on formation rock, downhole equipment, and in production tubing. This can damage reservoir porosity, as well as plug off production equipment and the well itself. Wells with excessive asphaltene deposition may incur high remediation costs but, more importantly, are exposed to levels of formation damage that can greatly shorten the productive life of the well. Crudes may only contain a few tenths of a percent of asphaltenes. Most crudes are less than 20% asphaltenes.
It is a common practice in the oil industry to control mineral scale deposition in reservoir rock by “squeezing” a chemical inhibitor into the reservoir through injection down a well. Scale inhibitors are water soluble charged materials that commonly adsorb to the reservoir rock and are produced back only very slowly, while leaching enough of a steady stream of chemical into the reservoir fluids to inhibit scale formation. Asphaltene inhibitors, however, are typically oil soluble nonionic materials that have little or no affinity to adhere and/or adsorb to reservoir rock, so squeezing an asphaltene inhibitor into the reservoir will not provide extended protection against asphaltene deposition because the inhibitor will be rapidly depleted as the well is produced. Squeeze methods of deposition control, while well known in the industry for other problems, therefore lack the proper chemical inhibitors to be useful in the case of asphaltene control in subterranean reservoirs.
It would be desirable if new methods and asphaltene inhibitor compositions which were capable of adsorbing onto reservoir rock to enough of an extent as to extend the effective term of inhibition of asphaltenes during hydrocarbon production.